In satellite communications, multiple access is generally thought of as the capability of a communications satellite to function as a portion of a communications link between more than one pair of remote communications terminals at the same time. For example, a multiple access communications satellite may provide a first communications link between a pair of satellites in medium Earth orbit (MEO) and a second communications link between a satellite in low Earth orbit (LEO) and a terminal on Earth.
Current multiple-access communications satellite systems include the Tracking and Data Relay Satellite System (TDRSS), which is a network of American communications satellites (each called a Tracking and Data Relay Satellite (TDRS)) and ground stations used by the National Air and Space Administration (NASA) for space communications. TDRSS was established in 1973 to provide continuous communications services to NASA's most critical LEO missions. Launches of TDRS spacecraft began in the 1980s and have continued through the present day. Most of these satellites are still operational today (with a number of them operating beyond their design life).
TDRSS communicates via radio-frequency (RF) channels in space and between space and Earth. Each satellite in the most recent generation of satellites provides ground reception rates of up to 300 kbits in the S-Band, 300 Mbit/s in the Ku-band, and 800 Mbit/s in the Ka-band using a pair of single-user, 5-meter dishes. Because these dishes are single-user dishes, usage for these channels is planned beforehand. Each satellite also includes an S-band phased array with 15 transmit elements and 32 receive elements to support multiple-access RF communications at a rate of up to 300 kbps per channel. This phased array enables unplanned, simultaneous reception of several signals and transmission of one signal.